1. Field of the Invention
The present invention relates to processes for monitoring plasma charging in semiconductor wafers. More particularly, the present invention relates to the use of optical emission spectroscopy in plasma etching and deposition processes.
2. Description of the Related Art
Semiconductor wafer fabrication involves a series of processes used to create semiconductor devices and integrated circuits (ICs) in and on a semiconductor wafer surface. Fabrication typically involves the basic operations of layering and patterning, together with others such as doping, and heat treatments. Layering is an operation used to add thin layers of material (typically insulator, semi-conductor or conductor) to the surface of the semiconductor wafer. Patterning is an operation that is used to remove specific portions of the top layer or layers on the wafer surface. Patterning is usually accomplished through the use of photolithography (also known as photomasking) to transfer the semiconductor design to the wafer surface.
In the wafer fabrication, plasma enhanced chemical vapor (PECVD) deposition and plasma etching are used respectively to deposit thin films and etch patterned wafers. During these plasma processes, metal lines (acting like an antenna structure) can collect charge from the plasma and transfer the charge to poly gates via metal interconnects. If the plasma charge voltage is high, such as, for example, exceeding 6 volts, the charge accumulated on the poly gates can damage the gate oxide. This results in a weak gate oxide and induces high leakage. Therefore continuous monitoring and reduction in plasma charging is critical for device manufacturing.
Conventional approaches to the monitoring of plasma charging include the use of wafer level reliability wafers (WLR) or EPROM wafers. In one approach, the wafer level reliability (WLR) wafers are processed through the production line. The WLR wafer has various types of metal antenna structures, which mimic those typically used in a manufacturing site. The wafers are tested upon fabrication completion, and the quality of gate oxide can be tested by measuring the charge breakdown voltage (QBD). Alternatively, the plasma charge can be tested by the threshold voltage (Vt) shift. But testing using WLR wafers to identify plasma charging damage is exceeding slow, sometimes taking several weeks or months to receive the data feedback. Moreover, the results depend on the quality of the gate oxide. Finally, results from individual plasma process steps interact with other plasma process steps, making it difficult to isolate the damage caused by a particular plasma operation.
Similarly, the EPROM wafer testing is a slow process, typically taking several days to yield useful data. EPROM test wafers EPROM wafers are also very expensive, the cost further increasing due to the test wafer""s susceptibility to plasma damage and the resulting need for frequent replacement EPROM test wafers.
Accordingly, it is desirable to provide a more effective method and apparatus for monitoring and reducing plasma charge damage.
To achieve the foregoing, the present invention provides methods and apparatus for determining plasma charge voltages and correlating plasma damage using optical emission spectra. The present invention provides an improved process for determining species contributing to plasma charging damage and using optical emission spectra for those species to optimize the plasma process or to control the plasma process. The present invention also provides an apparatus to monitor process variations using optical emission spectra. Selected optical emission peaks, such as determined using optical emission spectrometry techniques, correlate with plasma charging. These peaks may be measured in real time to optimize the plasma process to avoid plasma processing damage. Alternatively, the emission peaks may be used to monitor the plasma process drift to match a plasma process in one chamber with the same process in a different chamber.
In one embodiment, the present invention provides a method for determining process species associated with plasma charging damage. Optical emission spectra from a test wafer are measured. The plasma charging voltage detected by the test wafer is measured. The emission spectra are correlated with the plasma charging voltage to identify the species contributing to the plasma charging voltage.
In another embodiment, the present invention provides a method for adjusting plasma process parameters by monitoring the optical emission spectral output. A first optical emission spectrum correlated with plasma charging is measured. A plasma process parameter is adjusted in response to the measured first optical emission spectrum. In one embodiment, the first optical emission spectrum is measured also after the adjustment of the process parameter.